In 3GPP (3rd Generation Partnership Project) GSM (Global System for Mobile communications)/GPRS (General Packet Radio Service)/EDGE (Enhanced Data rate for GSM Evolution) wireless devices associated with a single SIM (subscriber identity module) paging blocks are typically regularly read at periodic well-defined positions when the device is turned on and registered to a network. The paging block reading enables the device to receive and answer to a call initiated by another party (i.e. a Mobile Terminated (MT) call).
The success rate of paging block readings is typically used to monitor the downlink quality. If the downlink quality is too bad, measures should typically be taken to find an alternative (e.g. cell re-selection and/or cell search).
As defined by the 3GPP GSM/GPRS/EDGE recommendations, the downlink quality monitoring may be based on a counter (DSC: Downlink Signaling Counter) in the wireless device. When the wireless device camps in a cell, the DSC is initialized to a specific value related to the paging periodicity as defined in the 3GPP specification (see e.g. 3GPP TS 45.008 Radio subsystem link control (Release 9) v.9.5.0 (2010 November)), section 6.5). The counter is updated at each paging block reading occasion by a value that is based on whether the paging reading was successful or not.
FIG. 1 illustrates a typical method 100 of such DSC handling (see also 3GPP TS45.008 §6.5). In step 110, it is determined whether or not the paging block reading was successful (e.g. if the paging block was correctly decoded). This check may typically be performed by CRC (cyclic redundancy check) or similar.
If the paging block reading was successful (YES path out of block 110) then the DSC is increased by 1 in step 120 and the method returns to step 110 for the next paging block reading. Typically, the DSC is not increased above a maximum value though. Then, if the DSC already has its maximum value, no increase is performed at step 120. The maximum value is typically equal to the specific value to which the DSC is initiated as described above.
If the paging block reading was not successful (NO path out of block 110) then the DSC is decreased by 4 in step 130. Typically, the DSC is not decreased below 0 though. Then, if the DSC already has a value less than 4, the DSC is just set to 0 at step 140.
Then, in step 140, it is determined if the DSC has reached the value 0. If it has not (NO path out of step 140), the method returns to step 110 for the next paging block reading. If DSC has reached the value 0 (YES path out of step 140), then a Downlink Signaling Failure (DSF) event is triggered in step 150, and the process returns to step 110. In good network coverage, the ordinary reselection procedure based on power level monitoring of the serving cell and of the neighboring cells leads to that the number of consecutive badly decoded paging blocks are typically kept at a very low level, and therefore the DSF event is avoided.
The initial value of the DSC is typically defined so that a DSF event is triggered if several consecutive paging blocks are badly decoded during 5 seconds.
The DSF event is used to force measures to be taken that may improve the downlink quality situation. For example, the DSF event may be used to (urgently) force a cell reselection or to trigger a cell search. In some situations, a HPLMN (Home Public Land Mobile Network) neighborhood cell coverage research may be triggered for example. If no new candidate serving cell is found, the SIM is considered to be in “No Service” (having no access to the HPLMN) or in “Limited Service” (having possible access to another PLMN for emergency calls).
When the wireless device is a dual or multi SIM device (e.g. Dual SIM Dual Standby—DSDS, typically having a single radio frequency module and a single baseband module), the situation regarding paging reading and its success is different than in the single SIM situation as will be described in the following. The problem will be exemplified via a DSDS device, but it should be noticed that similar problems arise for any multiple subscriber identity device not capable of providing simultaneous radio access to all of the subscriber identities.
For a dual SIM device, the paging block positions for each of the SIMs could be in conflict.
An example of such a situation is illustrated in FIG. 2A, where the paging block positions 201, 202, 203 of a first SIM 200 completely overlap the paging block positions 211, 212, 213 of a second SIM 210. Zooming in 215 on one of the paging block occasions shows that there is a complete overlap/collision between the 4 paging bursts 204 of the paging block position 201 of the first SIM 200 and the 4 paging bursts 214 of the paging block position 211 of the second SIM 210.
Another example of such a paging block conflict situation is illustrated in FIG. 2C, where the paging block positions 251, 252, 253 of a first SIM 250 partly overlap the paging block positions 261, 262, 263 of a second SIM 260. Zooming in 265 on one of the paging block occasions shows that there is a partial overlap/collision between the 4 paging bursts 254 of the paging block position 251 of the first SIM 250 and the 4 paging bursts 264 of the paging block position 261 of the second SIM 260. It is seen that the last 2 of the paging bursts 254 overlap/collide with the first 2 of the paging bursts 264.
There are also other overlap situations. For example a partial overlap could involve 1 or 3 of the paging bursts of the respective SIMs.
Since the multiple subscriber identity device is not capable of providing simultaneous radio access to all of the subscriber identities, it is not possible in paging occasion conflict situations to attempt or fulfill decoding of all paging blocks for all subscriber identities under the same conditions as in a single SIM situation.
Thus, the DSC handling and the DSF event occurrence may be affected for one or more of the multiple subscriber identities. In example situations, the DSF event may occur too soon or too late compared to a single SIM situation having the same conditions. Alternatively or additionally, the DSF event may occur even when it (according to the signaling quality conditions) should not have occurred at all. Yet alternatively or additionally, the DSF event may not occur at all even when it (according to the signaling quality conditions) should have occurred. This could cause the behavior of one or more SIMs to deviate from the 3GPP GSM recommendations. For example, a cell reselection, a cell search, a “No Service” procedure or a “Limited Service” procedure could wrongfully occur.
For further reference, see also e.g. 3GPP TS45.008 §6.5: “A downlink signaling failure shall result in cell reselection”. When a forced cell reselection is triggered, the serving cell is typically black listed in the UE (user equipment) for some time ahead (to avoid switching back too soon). If there is no other available cell in the neighboring cell list of the UE, it will typically try to find a new cell via a cell search procedure, and if no suitable cell is found the UE might typically go into a no service mode.
Hence, there is a need for methods and devices for downlink signaling counter handling in multiple subscriber identity devices that overcome or at least lessen the difference between the DSF event behavior compared to single subscriber identity devices.